1) Field of the Invention
The present invention relates generally to an apparatus that reduces interference on a coaxial cable or hybrid fiber-coaxial cable system. Specifically, the invention relates to an apparatus that attenuates return path noise in a transmission system using coaxial cable wherein the noise is caused by external radio frequency (RF) signals or impulsive RF noise which are picked up and carried on to the coaxial shield by the system ground wire. Once on the coaxial shield, the RF signals traverse the coaxial shield and, at shielding deficiencies in the cable or other components and devices, the undesired signals transfer to the coaxial center conductor. The RF signals then travel along the return path of the coaxial center conductor to the coaxial network provider site and interfere with the signals sent from the subscriber to the provider.
The apparatus of the present invention comprises an RF signal choke that is formed by wrapping a solid conductor, such as 12-gauge copper wire, around a ferrite toroidal form. An alternative method of creating an RF choke is to place the requisite ferrite material around the conductor at one or more points along the conductor path. Throughout this specification and claims xe2x80x9cRF Chokexe2x80x9d is defined to include either embodiment. The apparatus is placed in series with the system ground wire and the ground electrode or ground source. The apparatus significantly attenuates undesired external RF signals that are picked up and transmitted to the system via the ground wire thereby preventing the undesired RF signals from reaching the coaxial shield. The apparatus is provided in a compact housing with a terminal coupler and a conductor so that it is easily connected between a coaxial network ground wire or ground block and a grounding electrode at a subscriber site.
2) Description of the Related Art
The cable television industry, as well as other telecommunications industries, use certain segments of the radio frequency (RF) spectrum for transmission of signals in the upstream and downstream paths of communication in two-way RF distribution networks. The RF distribution networks can constitute coaxial cable networks or hybrid fiber-coaxial cable-based RF transmission systems. The invention herein is described as if it were installed on a coaxial cable system, however, it should be understood that the present invention can be utilized in any RF distribution network which provides for two-way signaling and which is grounded or bonded in at least one location on the RF distribution network.
Coaxial networks are commonly used to provide cable television and other programming from a provider to multiple subscribers. Coaxial networks have expanded dramatically over the years and are being used for a variety of different functions. Recent developments in the industry include using bi-directional coaxial and bi-directional hybrid-fiber-coaxial (xe2x80x9cHFCxe2x80x9d) networks to transmit computer data via the Internet to television sets and computers. In addition, cable television providers are expanding into telephony and other communication services. In such networks there are occasions where information from a subscriber site must be transmitted back to the system provider via the node, hub or headend.
For example, various pay-per-view events distributed on cable television can be ordered by a subscriber by indicating acceptance of the pay-per-view event on the cable set top terminal at the subscriber site. The subscriber is instructed to input a response into the cable set top terminal or television set at the subscriber site and the response is transmitted along the center conductor of the coaxial cable from the subscriber site to the provider. The information is then received and used or acted upon by the provider. In addition to pay-per-view type events, the uses for coaxial networks have expanded to include communications and other areas which require a two-way path for the transmission of information along the center coaxial conductor of a coaxial network. When a signal is transmitted from a subscriber to a provider, this is known in the industry as communication via the xe2x80x9cupstream path.xe2x80x9d
The upstream path""s frequency band typically ranges from 5 to 42 megahertz (MHz). This is known in the industry as the xe2x80x9cupstream frequency band.xe2x80x9d This frequency band corresponds to signals below the VHF television channel 2 in the United States NTSC system and therefore transmissions of signals on the upstream path do not interfere with the transmission of the primary signal along the downstream path (from the provider site to the subscriber sites).
One significant technical operational problem lies in the susceptibility of a coaxial network to RF interference from miscellaneous radiated or conducted ambient signals in the 5 MHz to 42 MHz band. These external signals can enter the coaxial network at one or more points and create interference or totally block the transmission of a return signal on the network. This effect is generally recognized and understood but is the source of significant concern to the industry. It is commonly referred to as xe2x80x9creturn path ingressxe2x80x9d because miscellaneous interfering signals of the 5 MHz to 42 MHz frequencies tend to ingress into the network and interfere with xe2x80x9creturnxe2x80x9d or xe2x80x9cupstreamxe2x80x9d signals. One prior art device is the Coaxial Cable Shield Filter which is fully described in U.S. Pat. No. 5,091,707, to Wollmershauser, et al. the disclosure of which is incorporated herein by reference. The Coaxial Cable Shield Filter described in the ""707 patent reduces interference caused by xe2x80x9coff-airxe2x80x9d RF signals that are conducted along the shielding of a coaxial cable. Such interference can cause problems with television reception at the subscriber sites if the conducted RF signals are allowed to enter a television or VCR tuner. The Coaxial Cable Shield Filter is designed to attenuate RF signals in a frequency range which interfere with signals traveling along the downstream path, namely, the primary signal sent from the provider site to the subscriber sites. The RF signals which the Coaxial Cable Shield Filter attenuates usually enter the coaxial network through direct pick-up (from a TV/VCR tuner) from external xe2x80x9coff-airxe2x80x9d RF signals (external sources) and are in the frequency tange corresponding to the frequencies used for downstream signals. When such RF signals are conducted along the shielding of the coaxial cable to the television or VCR tuner and are allowed to enter the tuner where they can mix with the downstream signals carried on the coaxial cable center conductor, they cause picture degradation and other problems in television reception.
The present invention is directed to solve a related problem but for which the coaxial cable shield filter is not suited, namely, interference which enters a coaxial network through its ground wire. Such interference is often caused by impulse noise caused by electrical devices within the subscriber site, as well as external RF signals from ham operators or other sources picked up by the wiring network at a subscriber site. Such interfering RF signals tend to cover a broad fluency range and encompass the frequency range of 5 MHz to 42 MHz.
Many household appliances use electrical contactors and similar electronic devices which produce impulsive noise. It is well known that as the current in an electromagnet rises and falls, its magnetic field grows and collapses. The moving field can induce a current in a nearby stationary conductor. Thus, when household appliances are operated, they can produce RF signals in electrical wing, metal casings of appliances, and metallic water pipes, etc. Such RF signals travel along the path of the conductor usually to a common ground source.
It is also known in the art that the electrical wiring in a typical home can act as an antenna and pick up RF signals in the air which are transmitted by remote sources such as ham radio operators, local television stations, etc. When the electromagnetic radiation comes into contact with the wiring system of a home, the RF impulse signals travel as a current along the wiring of the home and are usually conducted to the common ground shared by all of the electrical devices in the home.
In a typical subscriber site, all of the electrical appliances, communication devices, the power supply from a local utility and the coaxial cable from a cable television company are all bonded at a common grounding electrode. Usually, the common grounding electrode consists of a conductive rod driven into the ground near a home power supply panel and attached to the utility power supply ground or neutral. Another common grounding electrode used is the metallic cold water pipe in the home which comes through the earth into the home. Within the home, certain appliances have casings which are also grounded or bonded to the grounding electrode via the metallic cold water pipe. It is accepted practice for safety reasons to ground and/or bond certain appliances as well as telephone and coaxial networks to a common source so there is not a voltage potential difference between any single appliance and another appliance or electrical device in the home. In the home, impulse noise can be carried not only on a xe2x80x9cgroundxe2x80x9d wire but also on a xe2x80x9cneutralxe2x80x9d wire. Throughout this specification and claims the term xe2x80x9cground sourcexe2x80x9d or xe2x80x9cground reference sourcexe2x80x9d include all points which are electrically connected to the common ground. Also, the term xe2x80x9cground wirexe2x80x9d should also include xe2x80x9cneutral wirexe2x80x9d where applicable.
Removing any voltage potential difference between various appliances and devices in a home eliminates a shock hazard. Because the metal connector at the end of a coaxial cable is in electrical connection with the coaxial outer shield, and the connectors are readily accessible in a home, the coaxial shield must be grounded to the same ground source as other appliances in the home.
While the common bond increases safety, it provides a path for impulse noise and other undesired RF signals to enter any coaxial network sharing the common ground. Unfortunately, because all of these devices and the entire electrical wiring network in a home share a common connection with the coaxial network, a recurrent problem is that the RF signals and impulse noises are picked up and transmitted along this common ground to the coaxial network. The ground wire of a coaxial network is directly connected to the coaxial outer shield. Therefore, the RF signals can travel directly onto the coaxial cable shield of the coaxial network. Once on the coaxial cable shield, these RF signals traverse the shield and are carried by the shield until they are attenuated or dissipate.
Because a coaxial network has many connections and an extensive length between the subscriber site and the provider site, there are many places in the coaxial network where the coaxial shield can become weakened or become ineffective, or where devices or components that are installed, have marginal shielding effectiveness. It is at those locations that RF signals on the shield can traverse to the center conductor of the coaxial cable of a coaxial network. Once the RF signals reach the center conductor, they may interfere with or totally block the signals on the center conductor. Whether the RF signals interfere with the upstream signal or downstream signal depends on the frequency and amplitude of the interfering RF signals.
Most of the time, the RF signals which are picked up and sent to the coaxial shield via the coaxial network ground are problematic in the frequency band of 5 MHz to 42 MHz. Therefore, these RF signals tend to interfere with the upstream signals (from subscriber to provider). To eliminate these signals, a filtering apparatus needs to be placed in series with the ground wire of the coaxial network. However, the filtering apparatus cannot inhibit the capability of the ground wire to effectively function as a ground or safety issues could ensue as described above. For example, the Coaxial Cable Shield Filter described in the ""707 patent is a valuable invention, however, it is ineffective for use as a grounding conductor for a typical coaxial network
Other filters in the prior art used on coaxial networks are usually comprised of coaxial cable or other electrical components and therefore usually have coaxial cable connections for attachment directly to the coaxial network. Coaxial cable does not meet electrical code requirements for use as an acceptable ground wire. Therefore, when a coaxial network is installed at a subscriber site, the coaxial cable line from the provider is connected to a device known as a ground block. The ground block has an outer casing which is electrically connected to the coaxial outer shield of the coaxial cable. The ground block exterior casing is typically grounded or bonded using a 14-gauge or heavier copper wire to an appropriate ground or bonding source. The typical ground block also has two input/output female coaxial fittings which are inserted between the coaxial cable running from the ground block into the subscriber site. In this way, the coaxial cable is grounded without significantly disrupting the signal path.
In addition to attenuating RF signals, another problem which has developed is the effects of drop passive devices such as splitters and couplers that have ferrite transformers xe2x80x9cmagnetizedxe2x80x9d due to current flow in the ground path as fully set forth in the position paper of Scientific Atlanta on Drop Passive Non-Linearity created on or about Apr. 6, 1998, the disclosure of which is expressly incorporated herein by reference. The filter of the present invention tends to correct the effects of the ferrite magnetization when such magnetization is caused by impulsive or surge current in the ground path.
It is an object of the present invention to provide a filter which reduces interference on the return path of a coaxial network which also provides for an adequate grounding of the coaxial network near where a coaxial cable from the coaxial network typically enters a subscriber site.
What is needed to overcome the above-described RF signal return path noise and the disadvantages associated with known RF signal attenuation devices commonly used in a coaxial network is a commercially available, compact RF signal filter that can be readily and easily connected between the coaxial network ground block and an appropriate ground source. It is also an object of the present invention to provide an inexpensive, compact apparatus that is quickly and easily connected between a coaxial network ground block and the common ground at a subscriber site which attenuates RF signals in the frequency range used for return or upstream signals without loss of ground functionability.
The return path noise reducer of the present invention is generally comprised of a housing containing a filter device that attenuates the RF signals as they enter a coaxial network through its ground wire. The housing has a removable cover that is attached to the housing by a threaded fastener. The threaded fastener is received into a predrilled hole in the interior of the housing. The filter device is comprised of a ferromagnetic toroid ferrite form secured in the housing interior by epoxy or other equivalent fastening means. A length of solid conductor is wrapped several times around the toroid to create a choke. The solid conductor is of sufficient cross-section to enable it to also function as an adequate ground path for the coaxial network. An alternate embodiment could integrate the ground block and filter mechanism as one unit.
The opposite ends of the solid conductor are connected in series between the ground block of the coaxial network at a subscriber site and its ground source. In a preferred embodiment, a first end of the solid conductor is fastened to a terminal coupler. This terminal coupler is adapted to receive and hold in place a ground wire from the grounding electrode and to electrically couple the first end of the conductor to the grounding electrode. A ground wire from the grounding electrode is inserted into a first aperture in the terminal coupler through a first hole in the housing. A threaded fastener is threaded into a threaded receptacle in the top of the terminal coupler until it firmly engages the first end of the solid conductor and the ground wire from the grounding electrode. A second end of the solid conductor protrudes from the housing through a second hole in the housing and is at least several inches in length for attachment to the subscriber site ground block.
In practice, the filter of the present invention will attenuate RF signals as long as the toroid choke is in series along the ground path between the ground block and the source of the undesired RF signals. In the preferred embodiment, the filter is placed between the ground block and the utility power supply grounding electrode and it could be attached in any fashion and work as long as it is placed in series. However, the filter could be placed anywhere along the ground path before the power supply grounding electrode and it will attenuate RF signals. Also, numerous filters could be placed throughout the ground path. Thus, one could use a number of known methods to attach one end of the conductor to the ground block and the other end of the conductor to the grounding electrode with the conductor wrapped around the toroid between the two conductor ends. An alternative embodiment is the placement of several ferrite forms around the ground wire along the ground path.
In the preferred embodiment, the filter is connected between a coaxial outer shield and the grounding reference source. This is accomplished by attaching the filter of the present invention in series between the ground block and the grounding electrode. At the time of installation, repair or activation of the upstream communication path, of a coaxial network cable to a subscriber site, the filter can easily be attached in such a cable system. The ground wire traversing between the ground source and the ground block is disconnected from the ground block. The free end of the ground wire (which was originally attached to the ground block) is inserted through the first hole in the housing and into a first aperture in the coupling device. One then threads the male threaded bolt into the threaded receiver in the coupler through an access hole in the housing until the threaded bolt mates the free end of the ground wire and the first end of the solid conductor. One then attaches the second end of the solid conductor, which protrudes from the filter housing through a second hole, to the ground block.
By connecting the filter between the ground block and the ground reference supply, as described above, the RF signal choke is connected in series with the ground source and the coaxial cable shield. The choke acts as a filter that attenuates undesired RF signals which are carried by the coaxial network ground wire before the RF signals reach the outer shield of the coaxial cable of the coaxial network, but still functions as an adequate ground. Although the description herein is described at a subscriber site, it should be understood that the present invention can be used at any point along a transmission system to attenuate undesired RF signals which enter the system through a ground or bonding source.